The present invention relates to a liquid crystal display device; and, more particularly, the invention relates to a liquid crystal display device in which the reliability is enhanced by holding an optical sheet that is interposed between a liquid crystal panel and a backlight, which is arranged at a back surface of the liquid crystal panel, at a given position in such a way as to prevent a positional displacement of the optical sheet.
A liquid crystal display device, which is capable of generating a color display of high definition for a notebook type computer or a computer monitor, is provided with a light source for illuminating the liquid crystal panel from a back of the panel (a so-called backlight), and an optical sheet is interposed between the backlight and the liquid crystal panel for correcting the light provided from the backlight so that it has a given brightness distribution with respect to the liquid crystal panel.
The liquid crystal panel which constitutes this type of liquid crystal display device basically sandwiches a liquid crystal layer between two substrates, at least one of which is a transparent substrate, such as a glass plate or the like. This type of liquid crystal panel is roughly classified into a type which turns on and off given pixels by selectively applying voltages to various kinds of electrodes for forming pixels, which are formed on the substrate (simple matrix), and a type which forms the above-mentioned various kinds of electrodes along with active elements for selecting pixels and turns on and off given pixels by selecting the active elements (active matrix). Currently, due to the fact that the active matrix type liquid crystal panel has advantageous characteristics, such as high definition and high-speed display, the active matrix liquid crystal panel has been popularly accepted.
The conventional active matrix type liquid crystal display device employs a so-called vertical electric field type system in which an electric field is applied between pixel electrodes which are formed on one substrate and a common electrode which is formed on the other substrate so as to change the orientation direction of the liquid crystal layer (see Japanese Laid-open Patent Publication 309921/1988).
On the other hand, a so-called liquid crystal display device of the lateral electric field type (also referred to as “IPS type”) has been developed, in which the direction of the electric field applied to the liquid crystal layer is substantially parallel to the surface of the substrate. As an example of a liquid crystal display device of this lateral electric field type, a display device has been proposed which can obtain an extremely wide viewing angle by using comb-shaped electrodes at one of the two substrates (see Japanese Laid-open Patent Publication 21907/1988, U.S. Pat. No. 4,345,249).
In any one of the above-mentioned liquid crystal display devices, as an illumination light source of the liquid crystal panel, there is a side edge backlight, which is constituted of a light guide plate and a linear lamp, and a direct backlight, in which a plurality of linear light sources are directly installed at a back surface of the liquid crystal panel.
Particularly, the side edge backlight is constructed as follows. A linear lamp (cold cathode fluorescent lamp, for example) is arranged along at least one side edge of a light guide plate, which is constituted of a transparent plate, such as an acrylic plate. Light irradiated from the linear lamp is introduced into the light guide plate, and the path of the light is changed in the course of propagation inside of the light guide plate and is irradiated from the light guide plate. Then, the light is corrected to a given brightness distribution by means of an optical sheet, which has a laminated structure formed of a light diffusion sheet and a prism, and then the light illuminates the liquid crystal panel disposed above the optical sheet.
The liquid crystal display device is assembled into a so-called liquid crystal display module by integrating the liquid crystal panel and the backlight together with the optical sheet using an upper frame and a mold case (also referred to as a “lower frame”). Usually, the light guide plate which constitutes the backlight is fitted into the mold case which constitutes the lower frame, and, thereafter, the optical sheet is positioned on the mold case. Then, the liquid crystal panel is mounted on and is positioned on the optical sheet.
However, the optical diffusion sheet and the prism sheet, which constitute the optical sheet, are extremely thin film-like members. These film-like members must be positioned at a given position on the lower frame. Further, it is necessary to prevent a positional displacement and disengagement of the film-like members in the course of conveying the assembly during the manufacturing process or in the course of transporting it to an assembly plant for further assembly.
FIG. 17A and FIG. 17B are schematic diagrams illustrating an example of a conventional a structure for positioning and holding an optical sheet in a liquid crystal display device, wherein FIG. 17A is a cross-sectional view of a relevant part and FIG. 17B is a developed perspective view of the relevant part.
In FIG. 17A and FIG. 17B, MCA indicates a lower frame in the form of a resin mold which is configured to house a reflection sheet RFS, a light guide plate GLB, an optical sheet OPS, a liquid crystal panel PNL and the like. The reflection sheet RFS is mounted on the lower frame MCA, and the light guide plate GLB is fitted onto the reflection sheet RFS from above. The optical sheet OPS is positioned on the light guide plate GLB.
In this example, as shown in FIG. 17B, the optical sheet OPS is constituted of four film members in total, wherein optical diffusion sheets SPS are laminated to both sides of two prism sheets PRS whose groove directions cross each other. A projecting portion (lug) TAB is formed on one of the peripheries of the optical sheet OPS, and the optical sheet OPS is positioned by passing a columnar member (pin) PIN, which is mounted on the lower frame MCA, through a hole HOL formed in the projecting portion TAB. Thereafter, a cylindrical sleeve SB made of silicone or rubber is fitted on the columnar member PIN from above the columnar member PIN so as to fix the optical sheet, thus preventing removal or disengagement of the optical sheet OPS from the columnar member PIN.
Then, the liquid crystal panel PNL is assembled into the lower frame MCA and is then covered with an upper frame SHD. Subsequently, the upper frame SHD is fixed to the lower frame MCA so as to form an integrated liquid crystal display module.
The mounting position of the columnar member PIN is usually disposed at one of the peripheries of the lower frame MCA so as to avoid a side of the liquid crystal panel PNL on which a driver is mounted and a portion thereof where a liquid crystal filling and sealing opening is formed. A similar structure or a structure which simply holds the optical sheet loosely is provided at other peripheries of the lower frame MCA.
FIG. 18A and FIG. 18B are schematic views illustrating another conventional example of the positioning of an optical sheet in a liquid crystal display device and a holding structure thereof, wherein FIG. 18A is a cross-sectional view of a relevant part and FIG. 18B is a plan view of the relevant part. The same numerals as those used in FIG. 17A and FIG. 17B indicate identical functional elements.
In this example, U-shaped walls WL which surround projecting portions TAB formed on an optical sheet OPS from three directions are formed at four peripheries of a lower frame, and the projecting portions TAB formed on the optical sheet OPS are fitted into and seated in recessed portions formed by these U-shaped walls WL so as to provide for the positioning of the optical sheet OPS.
Further, there may be a method in which such U-shaped walls WL are formed only at two opposing peripheries or neighboring peripheries, or in which the U-shaped wall WL is formed at only one periphery and the optical sheet OPS and the lower frame are fixed to each other using an adhesive tape or the like at peripheries which are not provided with the U-shaped walls WL.
FIG. 19 is a developed perspective view illustrating a further conventional example of the positioning of an optical sheet in a liquid crystal display device and a holding structure thereof. In this conventional example, recessed portions DH are formed in front and back surfaces of a lower frame MCA. Columnar members PIN, similar to the columnar members PIN of the example described in conjunction with FIG. 17A and FIG. 17B, are formed such that they project at both front and back sides of the recessed portions DH. A projecting portion TAB, which is formed on the optical sheet OPS, is seated in the front-side recessed portion DH such that the columnar member PIN passes through a hole HOL formed in the projecting portion TAB. Thereafter, a clip CLM is fitted on the columnar members PIN which are projected at both front and back sides of the recessed portions DH.
This clip CLM is provided with an opening PR1 into which the front-side columnar member PIN is fitted from the lateral direction and an opening PR2 into which the back-side columnar member PIN is filled from the lower direction. The clip is formed of a metallic resilient material and resiliently fixes the optical sheet OPS to the lower frame MCA.
Examples of the above-mentioned structures are disclosed, for example, in Japanese Laid-open Patent Publication 281966/1999, Japanese Laid-open Patent publication 90361/1997 and Japanese Laid-open Patent Publication 258756/2000.